application

MODULE

application

MODULE SUMMARY

Generic OTP application functions

DESCRIPTION

In OTP, application denotes a component implementing
some specific functionality, that can be started and stopped as a
unit, and which can be re-used in other systems as well. This
module interfaces the application controller, a process
started at every Erlang runtime system, and contains functions
for controlling applications (for example starting and stopping
applications), and functions to access information about
applications (for example configuration parameters).

An application is defined by an application specification. The specification is normally located in an
application resource file called Application.app,
where Application is the name of the application. Refer to
app(4) for more information about
the application specification.

This module can also be viewed as a behaviour for an application
implemented according to the OTP design principles as a
supervision tree. The definition of how to start and stop
the tree should be located in an application callback module exporting a pre-defined set of functions.

Loads the application specification for an application into
the application controller. It will also load the application
specifications for any included applications. Note that
the function does not load the actual Erlang object code.

The application can be given by its name Application.
In this case the application controller will search the code
path for the application resource file Application.app
and load the specification it contains.

The application specification can also be given directly as a
tuple AppSpec. This tuple should have the format and
contents as described in app(4).

If Distributed == {Application,[Time,]Nodes},
the application will be distributed. The argument overrides
the value for the application in the Kernel configuration
parameter distributed. Application must be
the name of the application (same as in the first argument).
If a node crashes and Time has been specified, then
the application controller will wait for Time
milliseconds before attempting to restart the application on
another node. If Time is not specified, it will
default to 0 and the application will be restarted
immediately.

Nodes is a list of node names where the application
may run, in priority from left to right. Node names can be
grouped using tuples to indicate that they have the same
priority. Example:

Nodes = [cp1@cave, {cp2@cave, cp3@cave}]

This means that the application should preferably be started
at cp1@cave. If cp1@cave is down,
the application should be started at either cp2@cave
or cp3@cave.

If Distributed == default, the value for
the application in the Kernel configuration parameter
distributed will be used.

loaded_applications() -> [{Application, Description, Vsn}]

Types:

Application = atom()

Description = Vsn = string()

Returns a list with information about the applications which
have been loaded using load/1,2, also included
applications. Application is the application name.
Description and Vsn are the values of its
description and vsn application specification
keys, respectively.

permit(Application, Permission) -> ok | {error, Reason}

Types:

Application = atom()

Permission = boolean()

Reason = term()

Changes the permission for Application to run at
the current node. The application must have been loaded using
load/1,2 for the function to have effect.

If the permission of a loaded, but not started, application
is set to false, start will return ok but
the application will not be started until the permission is
set to true.

If the permission of a running application is set to
false, the application will be stopped. If
the permission later is set to true, it will be
restarted.

If the application is distributed, setting the permission to
false means that the application will be started at, or
moved to, another node according to how its distribution is
configured (see load/2 above).

The function does not return until the application is
started, stopped or successfully moved to another node.
However, in some cases where permission is set to true
the function may return ok even though the application
itself has not started. This is true when an application
cannot start because it has dependencies to other
applications which have not yet been started. When they have
been started, Application will be started as well.

By default, all applications are loaded with permission
true on all nodes. The permission is configurable by
using the Kernel configuration parameter permissions.

set_env/3 uses the standard gen_server timeout
value (5000 ms). A Timeout argument can be provided
if another timeout value is useful, for example, in situations
where the application controller is heavily loaded.

Warning

Use this function only if you know what you are doing,
that is, on your own applications. It is very application
and configuration parameter dependent when and how often
the value is read by the application, and careless use
of this function may put the application in a
weird, inconsistent, and malfunctioning state.

Starts Application. If it is not loaded,
the application controller will first load it using
load/1. It will make sure any included applications
are loaded, but will not start them. That is assumed to be
taken care of in the code for Application.

The application controller checks the value of
the application specification key applications, to
ensure that all applications that should be started before
this application are running. If not,
{error,{not_started,App}} is returned, where App
is the name of the missing application.

The application controller then creates an application master for the application. The application master is
the group leader of all the processes in the application.
The application master starts the application by calling
the application callback function Module:start/2 as
defined by the application specification key mod.

The Type argument specifies the type of
the application. If omitted, it defaults to temporary.

If a permanent application terminates, all other
applications and the entire Erlang node are also terminated.

If a transient application terminates with Reason == normal, this is reported but no other applications are
terminated. If a transient application terminates
abnormally, all other applications and the entire Erlang
node are also terminated.

If a temporary application terminates, this is reported
but no other applications are terminated.

Note that it is always possible to stop an application
explicitly by calling stop/1. Regardless of the type of
the application, no other applications will be affected.

Note also that the transient type is of little practical use,
since when a supervision tree terminates, the reason is set to
shutdown, not normal.

local is returned if only parts of the application is
being restarted (by a supervisor), or if the function is
called outside a startup.

If the process executing the call does not belong to any
application, the function returns undefined.

stop(Application) -> ok | {error, Reason}

Types:

Application = atom()

Reason = term()

Stops Application. The application master calls
Module:prep_stop/1, if such a function is defined, and
then tells the top supervisor of the application to shutdown
(see supervisor(3)). This means that the entire
supervision tree, including included applications, is
terminated in reversed start order. After the shutdown,
the application master calls Module:stop/1.
Module is the callback module as defined by
the application specification key mod.

Last, the application master itself terminates. Note that all
processes with the application master as group leader, i.e.
processes spawned from a process belonging to the application,
thus are terminated as well.

When stopped, the application is still loaded.

In order to stop a distributed application, stop/1
has to be called on all nodes where it can execute (that is,
on all nodes where it has been started). The call to
stop/1 on the node where the application currently
executes will stop its execution. The application will not be
moved between nodes due to stop/1 being called on
the node where the application currently executes before
stop/1 is called on the other nodes.

Performs a takeover of the distributed application
Application, which executes at another node
Node. At the current node, the application is
restarted by calling
Module:start({takeover,Node},StartArgs). Module
and StartArgs are retrieved from the loaded application
specification. The application at the other node is not
stopped until the startup is completed, i.e. when
Module:start/2 and any calls to
Module:start_phase/3 have returned.

Thus two instances of the application will run simultaneously
during the takeover, which makes it possible to transfer data
from the old to the new instance. If this is not acceptable
behavior, parts of the old instance may be shut down when
the new instance is started. Note that the application may
not be stopped entirely however, at least the top supervisor
must remain alive.

See start/1,2 for a description of Type.

unload(Application) -> ok | {error, Reason}

Types:

Application = atom()

Reason = term()

Unloads the application specification for Application
from the application controller. It will also unload
the application specifications for any included applications.
Note that the function does not purge the actual Erlang
object code.

Removes the configuration parameter Par and its value
for Application.

unset_env/2 uses the standard gen_server
timeout value (5000 ms). A Timeout argument can be
provided if another timeout value is useful, for example, in
situations where the application controller is heavily loaded.

Warning

Use this function only if you know what you are doing,
that is, on your own applications. It is very application
and configuration parameter dependent when and how often
the value is read by the application, and careless use
of this function may put the application in a
weird, inconsistent, and malfunctioning state.

Returns a list with information about the applications which
are currently running. Application is the application
name. Description and Vsn are the values of its
description and vsn application specification
keys, respectively.

which_applications/0 uses the standard
gen_server timeout value (5000 ms). A Timeout
argument can be provided if another timeout value is useful,
for example, in situations where the application controller
is heavily loaded.

EXPORTS

This function is called whenever an application is started
using application:start/1,2, and should start
the processes of the application. If the application is
structured according to the OTP design principles as a
supervision tree, this means starting the top supervisor of
the tree.

StartType defines the type of start:

normal if it's a normal startup.

normal also if the application is distributed and
started at the current node due to a failover from another
node, and the application specification key start_phases == undefined.

{takeover,Node} if the application is
distributed and started at the current node due to a
takeover from Node, either because
application:takeover/2 has been called or because
the current node has higher priority than Node.

{failover,Node} if the application is
distributed and started at the current node due to a
failover from Node, and the application
specification key start_phases /= undefined.

StartArgs is the StartArgs argument defined by
the application specification key mod.

The function should return {ok,Pid} or
{ok,Pid,State} where Pid is the pid of the top
supervisor and State is any term. If omitted,
State defaults to []. If later the application
is stopped, State is passed to
Module:prep_stop/1.

This function is used to start an application with included
applications, when there is a need for synchronization between
processes in the different applications during startup.

The start phases is defined by the application specification
key start_phases == [{Phase,PhaseArgs}]. For included
applications, the set of phases must be a subset of the set of
phases defined for the including application.

The function is called for each start phase (as defined for
the primary application) for the primary application and all
included applications, for which the start phase is defined.

This function is called by an application after a code
replacement, if there are any changes to the configuration
parameters.

Changed is a list of parameter-value tuples with all
configuration parameters with changed values, New is
a list of parameter-value tuples with all configuration
parameters that have been added, and Removed is a list
of all parameters that have been removed.